Adults with asthma who had been enrolled in an HMO for at least a year were requested to complete a questionnaire about their health status. Approximately 25% of the 1,461 participants responded positively to "Does your current work environment make your asthma worse?" and were classified as having workplace exacerbation of asthma. Those with workplace exacerbation were more likely to have never attended college, be current or former smokers, have a history of other respiratory diseases, have missed work or usual activities at least one day in the past for weeks, and report their asthma was moderate, severe, or very severe. Percentages with workplace exacerbation of asthma were highest for mining and construction (36%), wholesale and retail trade (33%), and public administration (33%), and lowest for educational services (22%), finance, insurance, and real estate (22%), and non-medical and non-educational services (18%). Future studies are needed for objective validation of self-reported workplace exacerbation, and to follow subjects prospectively to clarify the temporal sequence of workplace exacerbation and asthma severity, and how other respiratory conditions and smoking might contribute to work-related worsening of asthma.